The present invention relates generally to refrigeration systems using economizers, such as those employed for chiller applications.
Some refrigeration and air conditioning systems rely on chillers to reduce the temperature of a process fluid, typically water. In such applications, the chilled water may be passed through downstream equipment, such as air handlers, to cool other fluids, such as air in a building. In typical chillers, the process fluid is cooled by an evaporator that absorbs heat from the process fluid by evaporating refrigerant. The refrigerant is then compressed by a compressor and transferred to a condenser. In the condenser, the refrigerant is cooled, typically by air flow and recondenses into a liquid. Air cooled condensers typically comprise a condenser coil and a fan that induces airflow over the coil. In some conventional designs, economizers are utilized in the chiller design to improve performance. In systems that employ flash tank economizers, the condensed refrigerant may then be directed to the flash tank where the liquid refrigerant at least partially evaporates. The vapor may be extracted from the flash tank and redirected to the compressor, while liquid refrigerant from the flash tank is directed to the evaporator, closing the refrigeration loop.
In a conventional system of this type, a flow control valve, which may be referred to as a feed valve, is provided in the conduit between the condenser and the flash tank. Flow into the flash tank is typically controlled in a closed-loop manner based upon the flash tank level. A drain valve, used to extract liquid from the flash tank also may be controlled in a closed-loop manner, typically based upon superheating of the refrigerant leaving the evaporator. Superheating of the refrigerant refers to heating above the boiling point.
However, applications exist for other types of evaporators that are generally incapable of superheating the refrigerant. Certain advantages may flow from the use of such evaporators in conjunction with flash tanks. Evaporators of this type may include shell-side evaporators, such as falling film evaporators, in which the refrigerant is sprayed over tubes through which the second process fluid (e.g., water) circulates. Other evaporators with shell-side evaporation include flooded evaporators or hybrids of falling film and flooded evaporator designs. The evaporation of the refrigerant on the outside of the tubes cools the second process fluid. Because no superheating occurs in the refrigerant outflow from the evaporator, conventional techniques for regulating levels in a flash tank based upon superheating are not available.
There is a need, therefore, for improved techniques for controlling refrigerant levels and flow in heating, ventilating, and air conditioning systems that can make use of tube-side condensers and shell-side evaporators with flash tanks.